The present invention relates to a probe for measuring light scattering which performs light scattering measurements by illuminating a sample with light and detecting light scattered from inside a scattering volume.
An instrument for measuring light scattering refers to an instrument that measures fluctuation, or change with time, in intensity of scattering light caused by motion of particles present in a fluid (Brownian Motion), thereby determining the diffusion coefficient and hydrodynamic size of the particles.
In a conventional instrument for measuring light scattering, measurements are carried out by illuminating a cell shaped as a cylinder or a rectangular parallelepiped filled with a sample fluid with a laser beam through a lens, passing scattered light emitted from the sample through a light-receiving system in which the observation volume is limited by a pinhole or the like, and measuring the scattered light by means of a photodetector such as a photo multiplier.
In the above instrument for measuring light scattering, the light path length in the cell of the sample fluid is long. Accordingly, as the concentration of particles in the solution increases, scattering by the scattered light, or multiple scattering, occurs in the cell, making it impossible to obtain accurate information on the scattered light.
In order to overcome this inconvenience, there has been proposed a structure of a probe for measuring light scattering (R. R. Khan, H. S. Dhadwal, and K. Suh, Applied Optics 33(25), 1994), in which an end of a light input optical fiber and an end of a light receiving optical fiber are disposed in the cell at a predetermined angle in close proximity to each other. This probe for measuring light scattering allows the observation volume to be small, thereby precluding the problem of multiple scattering.
In the above mentioned probe for measuring light scattering, a micro lens or a graded-index fiber for bringing the focus of each optical fiber onto the intended position is disposed at the end face of each of the light input fiber and the light receiving fiber. Accordingly, high accuracy is required for positioning each optical fiber at each lens, making it difficult to keep the product quality at a practical level.
The basic structure of an optical fiber comprises a core that propagates light, and a cladding with a small refractive index surrounding the core, which is further covered with a resin coating for protection.
Relating to this, an invention has been disclosed as PCT International Publication No. WO00/31514, in which the cladding is exposed by removing the resin coating around the optical fiber, or the core is exposed by removing the cladding as well so that the distance between the end faces of the cores is made as small as possible so as not to be affected by multiple scattering.
However, in such a structure with the cladding or the core being exposed, the following problems arise: it is impossible to keep the strength of the optical fibers, adjustment of the positions of the cores is difficult, and light leaks from the cladding.
It is therefore an object of this invention to provide a probe for measuring light scattering that is easy to produce, and capable of measuring intensity of scattered light with high accuracy and high reliability.
A probe for measuring light scattering in accordance with the present invention comprises a light input optical fiber for transmitting light for illuminating a sample therewith, and a scattered light measuring optical fiber for collecting and transmitting scattered light. An end portion of each of the optical fibers is covered with a ferrule, an end of the ferrule being cut into the shape of a truncated cone such that a part of or the entire end face of the optical fiber remains. The ferrules are held by the probe for measuring light scattering in such a manner that the end faces of the optical fibers are disposed to be adjacent to each other at a predetermined angle with a predetermined distance in between.
Being arranged as above, the end portions of the optical fibers can be reinforced and protected by the ferrules even when they have poor strength. In addition, the distance and angle between the end faces of the optical fibers can be easily maintained by holding the ferrules. The ferrules also prevent light from leaking from the claddings.
Furthermore, as a result of cutting the ends of the ferrules into the shape of a truncated cone, bubbles in the sample fluid are less likely to adhere to the ends of the optical fibers. Also, the distance between the end faces of the optical fibers can be reduced in this arrangement as compared with cases where the ferrules are not tapered.
The angle and distance between the end faces of the optical fibers are preferably adjustable through the ferrules by an adjustment member. By this arrangement, it is possible to control the scattering volume to be a desired volume.
This adjustment can be performed precisely and easily by the use of screws.
The optical fibers are preferably single-mode optical fibers. This allows measurements to be performed in a condition where the coherence is improved.
The ferrules preferably have cladding portions of the optical fibers inserted therein excluding coating portions of the optical fibers. This makes it possible to taper the ends of the ferrules such that the cladding portions or the cladding portions and core portions inside thereof are cut obliquely. Accordingly, reducing the distance between the end faces of the optical fibers can be easily accomplished.